This invention relates to electronic substrates, and more particularly, relates to ceramic and organic substrates having multilayer features thereon for electronics packaging applications and a method for forming such multilayer features.
Glass, ceramic and glass ceramic (hereafter just ceramic) and organic structures are used in the production of electronic substrates and devices for electronics packaging applications. Many different types of structures can be used. For example, a multilayered ceramic circuit substrate may comprise patterned metal layers which act as electrical conductors sandwiched between ceramic layers which act as insulators. Organic substrates, also coalled printed circuit boards, may be single layer or multilayer material (such as fiberglass-impregnated epoxy) and contain electrical conductors. The substrates are designed with termination pads for attaching semiconductor chips, capacitors, resistors, connection leads, pins, solder balls, solder columns etc. Interconnection between buried conductor levels in ceramic substrates can be achieved through vias formed by metal paste-filled holes in the individual ceramic layers (called greensheets) formed prior to lamination, which, upon sintering will become a sintered dense metal interconnection of metal based conductor. In the case of organic substrates, interconnection between conductor levels is by, for example, plated through hole vias.
The termination pads are often multi-layered stacks of metallization and are conventionally produced with multiple screenings, with the underlying layer being screened and dried before application of another mask and screening and drying of the next layer. Greenstein U.S. Pat. No. 4,025,669, Siuta U.S. Pat. No. 5,202,153, and Knickerbocker et al. U.S. Pat. No. 5,293,504, the disclosures of which are incorporated by reference herein, are examples where multiple screenings have been utilized to obtain either a thicker layer or a multiple layer stack of metallization.
While the prior art shows the individual layers of the stack to be perfectly aligned with every other layer in the stack, the reality is very far from this ideal case. For example, Natarajan et al. U.S. Pat. No. 5,639,562, the disclosure of which is incorporated by reference herein, shows a two layer composite metal pad with both layers perfectly aligned.
Gaynes et al. U.S. Pat. No. 5,565,033, the disclosure of which is incorporated by reference herein, discloses a process for making thicker layers of solder pastes and conductive adhesives. Gaynes et al. recognizes the disadvantages of multiple screenings as contamination between successively screened layers and the time associated with two passes through screening and drying.
In practice, the individual layers may be shifted from the layer above or below it. Mitani et al. U.S. Pat. No. 4,324,815, the disclosure of which is incorporated by reference herein, recognizes the positional error that can occur with each printing step. As disclosed in Mitani et al., the bottom layer could be made larger than the top layer so that the top layer is "captured" by the bottom layer.
However, with the trend to increasing the density of the termination pads (and reducing the spacing between adjoining termination pads), it is no longer possible to oversize the bottom layer to capture the top layer. For example, a typical pin grid array substrate has a nominal pad diameter of 1.5 mm, an interpad space of 0.3 mm and a pad tolerance of +0/-0.220 mm. This is to be compared with a typical column grid array substrate which has a nominal pad diameter of 0.800 mm, an interpad space of 0.200 mm and a pad tolerance of +/-0.050 mm.
Accordingly, it is a purpose of the present invention to have an improved process for producing multilayer stacks of metallization on a ceramic article and/or organic article.
It is another purpose of the present invention to have an improved process for producing multilayer stacks of metallization on a ceramic article and/or organic article which eliminates the positional errors which heretofore have been inherent in multiple screenings of metallization.
It is yet another purpose of the present invention to have an improved process for producing multilayer stacks of metallization for use as termination pads, lines and other features.